Recently, the research and development of so-called high density optical disk in which a recording density of an optical information recording medium (optical disk) is increased by using a blue laser light of wavelength of about 400 nm and a storage capacity is increased, are advanced.
As a standard of the high density optical disk, for example, a standard in which an image side numerical aperture (NA) of an objective lens is about 0.85, a protective substrate thickness is about 0.1 mm, or a standard in which NA and the protective substrate thickness are controlled to about 0.65 and 0.6 mm, which are same degree as those of the conventional DVD (Digital Versatile Disk), is well-known. In the following description, the high density optical disk in which NA is about 0.65, and the protective substrate thickness is about 0.6 mm, is expressed as “HD-DVD (High Density DVD)”.
Various technologies relating to the optical pick-up apparatus having the interchangeability of such a high density optical disk with the optical disk such as DVD or CD (Compact Disc) which is conventionally widely used, are proposed.
Hereupon, wavelengths of light fluxes used for HD-DVD/DVD/CD, λ1/λ2/λ3 are respectively about 400 nm/650 nm/780 nm, and protective substrate thickness t1/t2/t3 are respectively about 0.6 mm/0.6 mm/1.2 mm.
Then, in order to attain the compatibility among 3 kinds of optical disks of HD-DVD/DVD/CD, it is necessary that, while the light amount of the light flux used for each of optical disks is preserved, the spherical aberration generated due to the difference of the wavelengths or protective substrate thickness is compensated, and a technology by which the diffractive structure is provided on the optical surface of the optical element constituting the optical pick-up apparatus, is disclosed in, for example, Japanese Un-examined Patent Application Publication No. 2002-298422.
Ordinarily, because, for recording and/or representing HD-DVD, more light amount is required than for CD or DVD, there are many cases in which, in the diffraction light of wavelength λ1, the diffraction order which is the maximum diffraction efficiency, and the blazed wavelength are selected.
In contrast to that, on DVD, because the protective substrate thickness is the same as HD-DVD (t1=t2=0.6 mm), the spherical aberration (A) is generated due to only a case where the wavelength is different. When this spherical aberration (A) is going to be compensated by using the diffraction effect, it is compensated by the difference of wavelength x diffraction order between HD-DVD and DVD.
In addition to that, when the transposition with CD is also considered, due to a case where the wavelength λ1 of the light flux for HD-DVD is about a half of wavelength λ3 of the light flux for CD, for example, when n-order diffraction light of the wavelength λ1 has the maximum diffraction efficiency, (n/2)-order diffraction light of the light flux of the wavelength λ3 has the maximum diffraction efficiency. Then, when the compatibility is attained by the ratio of the diffraction order of this wavelength λ1 and the wavelength λ3 (2:1), in CD, the diffraction order light of the wavelength λ3 has the maximum diffraction efficiency, on the one hand, the spherical aberration (B) (B>A) generated due to a case where the protective substrate thickness is different between HD-DVD and CD, can not be corrected by the difference of wavelength x diffraction order between HD-DVD and CD.
As described above, in the production of the optical pick-up apparatus having the interchangeability among 3 kinds of optical disks of HD-DVD/DVD/CD, there is a problem that the compatibility of the securement of the light amount of each light flux with the correction of spherical aberration is difficult. Also in the above-described Patent Document, the efficiency of 3rd order light of the high density optical disk (HD-DVD) using the light flux near the wavelength 400 nm, is about 73%, which is a value in which it can not always be said that it is sufficient for using for recording of the information.